InstantForum.NET v2.9.0SQLServerCentralhttp://www.sqlservercentral.com/Forums/notifications@sqlservercentral.comWed, 22 May 2013 17:55:10 GMT20http://www.sqlservercentral.com/Forums/Topic654593-360-1.aspxWhat showed 4 logical reads?[code]set statistics io ongoselect * from t where id<='00001'[/code]Results:Table 't'. Scan count 1, logical reads 2, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.Wed, 11 Feb 2009 04:00:38 GMTGilaMonsterhttp://www.sqlservercentral.com/Forums/Topic654593-360-1.aspxI create ths script as following:CREATE TABLE [dbo].[t]( [id] [char](6) NOT NULL, [b] [varchar](8000) NOT NULL, CONSTRAINT [pk_t] PRIMARY KEY CLUSTERED( [id] ASC)insert into t values('00001',REPLICATE('A',4000) )insert into t values('00002',REPLICATE('B',4000) )insert into t values('000021',REPLICATE('E',4000) )insert into t values('00003',REPLICATE('C',4000) ) insert into t values('00004',REPLICATE('D',4000) )OK,Let's dump index internal: dbcc ind(0,'t',1)PageFID PagePID IAMFID IAMPID ObjectID IndexID PartitionNumber PartitionID iam_chain_type PageType IndexLevel NextPageFID NextPagePID1 1989 NULL NULL 1609772792 1 1 72057594053459968 In-row data 10 NULL 0 01 1988 1 1989 1609772792 1 1 72057594053459968 In-row data 1 0 1 19941 1992 1 1989 1609772792 1 1 72057594053459968 In-row data 2 1 0 01 1993 1 1989 1609772792 1 1 72057594053459968 In-row data 1 0 0 01 1994 1 1989 1609772792 1 1 72057594053459968 In-row data 1 0 1 1993 Page id=1992 is a root page ,DBCC traceon(3604)dbcc page(0,1,1992,3) output like this:FileId PageId Row Level ChildFileId ChildPageId id (key) KeyHashValue1 1992 0 1 1 1988 NULL NULL1 1992 1 1 1 1994 00002 NULL1 1992 2 1 1 1993 00004 NULL my question is when I isse the following sql: select * from t where id<='00001'sql server report it need four logical reads.I think it only need read index root page(pageid=1192) and the left leaf index page(pageid =1988) ,so it need only two logical reads. who can explain it why it need four logical reads. Wed, 11 Feb 2009 03:12:08 GMTlovexueer